Fuel 85 (2006) 1550–1558 www.fuelfirst.com
Volatilization of mercury, arsenic and selenium during underground coal gasification Shuqin Liu a,*, Yongtao Wang a, Li Yu a, John Oakey b a
School of Chemistry and Environmental Engineering, China University of Mining and Technology (Beijing), T-11, Xueyuan Road, Haidian District, Beijing 100083, China b Power Generation Technology Centre, Cranfield University, Cranfield, Bedfordshire MK43 0AL, UK Received 27 March 2005; received in revised form 9 December 2005; accepted 14 December 2005 Available online 19 January 2006
Abstract Mercury, arsenic and selenium are trace elements well-known for their high volatility in underground coal gasification (UCG) which can lead to environmental and technical problems during gas utilization. In this paper, the volatilization of mercury, arsenic and selenium from coal in a seam during the process of UCG were investigated, based on comparison of their volatility during the transformation of coal to char and the conversion of char to ash. Three types of coal were involved in this study. The results indicate that the volatility of mercury, arsenic and selenium during UCG in the seam follows the sequence of HgOSeOAs. Mercury and selenium show volatility higher than 90% from coal to ash. The volatility of arsenic is lower than 60% as confirmed by arsenic enrichment in UCG ash. Arsenic volatilization during UCG is also enhanced by increasing temperature, which is different from the result during the combustion of crushed coal. Coal type has obvious effect on element volatilization. The higher the coal reactivity, the easier is the evaporation of the elements from coal in the seam. At the same time, thermodynamic equilibrium calculations using MTDATA program were performed to predict the possible species in UCG gas. With regard to UCG gas at the production well, mercury presents as Hg(g), H2Se(g) is the main gaseous species of selenium, whereas arsenic occurs in condensed phase as As2S3 and As. The effect of the pressure on the equilibrium composition of the gas results in major changes of the proportions of the species. High pressure leads to the formation and enhancement of the reduced species and increases the condensation temperature of the volatile elements. q 2006 Elsevier Ltd. All rights reserved. Keywords: Underground coal gasification; Trace elements; Volatilization
1. Introduction Coal combustion and gasification are potential sources of hazardous, environment-unfriendly trace element emissions if inadequate gas cleaning is installed. Being the highly volatile elements, mercury, arsenic and selenium are released from coal during the chemical and physical changes involved and become distributed mainly in the gas phase, which will be discharged into the atmosphere if not removed before. Therefore, it is necessary to understand the behaviour of these volatile elements during coal combustion and gasification so as to control and reduce their pollution. Reported studies on the behaviour of mercury, arsenic and selenium during coal combustion are quite extensive, in line with the wide industrial use of coal boilers [1,2], while relatively few papers describe * Corresponding author. Tel.: C86 10 62331897. E-mail address: [email protected]
0016-2361/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2005.12.010
their behaviour during the process of gasification. Research on the partitioning of mercury, arsenic and selenium during coal gasification, especially in the condition of air-blown fluidized bed gasification, have been conducted [3–5]. Most of this research is based on thermodynamic equilibrium calculations as the experimental data are either unavailable or not suitable for publication. Moreover, the calculations are always conducted at medium or high pressures [6–8]. UCG involves the conversion of coal in the seam into a combustible fuel gas thus avoiding the costs of mining and transportation. It is potentially a clean method of coal conversion into a high-energy fuel gas and may be adopted to replace small coal-fired power plants and to maximise coal resources ; this is partly, achieved through its implementation with unminable coal seams or those which have geological problems. It has been widely tested in China and used at commercial scale; both low and high pressure variants are being considered. UCG fuel gas can be used for power generation, hydrogen production and as a chemical feedstock. Gas turbine power generation combined with underground coal gasification is one possible approach to clean coal utilization
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which has been supported by the Department of Trade and Industry of UK in the past few years. The fuel gas produced from UCG process will inevitably be mixed with trace elements, especially the highly volatile ones such as mercury, arsenic and selenium, as a result of the high temperatures involved in the gasification process . However, UCG fuel gas cools as it passes through the production well and a significant proportion of the contaminants will deposit or condense in this transit. Those trace elements that do exit from the production well will be discharged into the atmosphere through direct combustion and will additionally cause deposition and corrosion when the gas is used for gas turbine power generation. With the widespread commercialization of UCG in China and increased attention in Europe, it is necessary to understand the behaviour of these trace elements during UCG in order to be able to assess the risk of pollution and to develop suitable systems for their reduction. It may be expected that the volatilization of mercury, arsenic and selenium may be a little different in UCG than during surface gasification as it involves the combustion and gasification of the coal in the seam. Up to now, no literature about the behaviour of trace elements during UCG has been reported. In this paper, the evaporation of mercury, arsenic and selenium during UCG process have been investigated based on their levels in coal and their volatility in the UCG process. Also, thermodynamic equilibrium calculations were used in this work to predict the possible gaseous species and condensed phases occurring in UCG fuel gas as it cools. The lower pressure for UCG applied to shallow coal seams in China was considered and the effect of pressure on equilibrium compositions was also studied. The results produced may help to establish a suitable method for their removal and capture. 2. Experimental 2.1. Simulation test of UCG and sampling 2.1.1. Construction of the simulated coal seam UCG ash and char have been prepared in simulation tests conducted in the UCG central laboratory of the China University of Mining and Technology in Beijing. Coal samples of Chinese lignite, bituminous coal and anthracite were used in this study provided by the coal mines at Dayan, Xinwen, and Xiyang, respectively. Proximate and ultimate analyses of these coals (on air dry basis) are shown in Table 1. Table 1 Proximate and ultimate analyses of coals (on air dry basis) Sample
Proximate analysis (wt%)
Ultimate analysis (wt%,ad)
Lignite 32.83 10.91 24.89 44.49 3.34 Bituminous coal 9.77 6.69 35.90 65.56 3.85 Anthracite 2.10 10.40 6.88 80.80 2.82
7.07 0.90 13.43 0.27 2.17 1.02
0.46 0.43 0.69
Fig. 1. Diagram of the UCG model gasifier.
A diagram of the UCG model gasifier used for these tests is shown in Fig. 1. A firebrick layer of 300 mm was used as the upper insulation layer. Below this, block coal, rock, sand, and clay were located in layers to simulate in situ coal seam conditions, including the dip angle of the coal seam. Rock samples were placed in above and below the coal layer to simulate roof rock and bottom rock. The bottom rock was placed according to the dip of the in situ coal seam (lignite, 158, bituminous coal, 208, and anthracite 228). The simulated coal seam was constructed using processed block coal samples with size of 500!500!500 mm3. A mixture of fine coal and cement was used to fill the gap between the block coals to form a continuous coal seam with a lateral length of 4500 mm, depth 500 mm, and incline length 1500 mm. A gasification tunnel with diameter of 150 mm was bored along the lateral direction, 150 mm away from the lower edge of the coal seam. Four vertical boreholes with diameter of 80 mm were drilled though the rock and coal layer to connect with the gasification tunnel, and steel pipes were located in these and connected to external gas pipes. These boreholes include an initial injection hole, a production hole, and auxiliary holes, which can be used for moving the oxygen supply and gas production exit as shown in Fig. 1. The final stage of construction was to prepare the compacted clay layer. Electric furnace heating wires were set in the gasification tunnel by the side of the injection borehole, together with one kilogram of crushed coal for ignition. 2.1.2. Gas supply and monitoring system The gasifying agents, air and oxygen were supplied by an air compressor and an oxygen tank (98%), respectively; steam was produced by a small steam boiler. Flows of all the gases were monitored by vortex flowmeters. 96 NiCr–NiSi thermocouples were laid in the coal seam, in 16 columns and 6 rows, to determine the position of the fire front and its development along the lateral and the incline. Four manometers were installed on the pipes connected to the boreholes and another two were set on to measure the pressure of the gasifier. The UCG gas produced was pumped away and connected to a gas chromatograph for automatic analysis every 30 min. All the data were transferred to the control computer for real-time monitoring.
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2.1.3. Operating procedure The electrical source was switched on to ignite the crushed coal and the nearby coal seam. After ignition, air (10–15 m3/h) was injected and the coal on the side of injection borehole began to burn, and a high temperature profile gradually formed. Then the air was switched to oxygen (5–6 m3/h) to enhance the combustion. The combustible gas began to form with the release of the volatile content from the coal seam. When the high temperature zone was above 900 8C, steam was carefully introduced. The steam flow was adjusted in the range of 5–20 m3/h to find the suitable steam/oxygen ratio and to maximise the amount of combustible gas, including carbon monoxide, hydrogen and methane based on on-line gas analysis. Following this, stable gasification proceeded at a controlled steam/oxygen ratio. With the gasification of the coal seam, the fire front moves forward and along the coal seam incline. When the fire front moved away from the injection hole, gas quality began to decrease. Also it was observed from the temperature profile that the velocity of the fire (gasification velocity) slowed as the cavity grew in size. In response, the steam flow was stopped and the valve switched to supply oxygen through the auxiliary borehole approaching the fire front. After the high temperature zone developed beside the auxiliary hole, steam was introduced again and gasification resumed. When the fire front reached the production hole, oxygen supply was reversed (so that oxygen was supplied through the production hole and gas production from the original injection hole) in order to sustain the gasification process. Moving the oxygen supply was a necessary part of the UCG process to ensure the progressive combustion and gasification of the fixed coal seam, but this leads to fluctuations of the gas composition and gasification pressure (0.10–0.12 MPa) in the experimental conditions). The same basic operation procedure was used for all three coals, with same variation in response to the data from real-time monitoring. 2.1.4. Sampling In the UCG model gasifier, the coal seam was gasified to produce fuel gas while ash and a small quantity of char were left behind. After each test, the gasifier was excavated to investigate cavity growth; UCG ash and remaining char were collected along the gasification tunnel. Samples were collected and reduced to 1 kg by quartering, and then ground and sieved for analysis. 2.2. Analysis of mercury, arsenic and selenium content in coal, char and ash For coal samples, wet digestion by a mixture of HNO3– H2SO4–V2O5 was conducted for mercury detection. Dry digestion using aldrosol was adopted for arsenic detection in coal samples and HNO3–HF–HClO4 wet digestion for selenium analysis. Other samples of char and ash were digested with a 5:1(V/V) mixture of hydrofluoric and perchloric acids. The concentration of mercury, arsenic and selenium were analyzed by atomic fluorescence spectrophotometer (AFS-810).
2.3. Sequential chemical extraction of trace elements in coal (1) Exchangeable species. one gram of the sample was extracted at room temperature for 1 h with 15 ml of MgCl2 solution (1 mol/l, pH 7.0) with continuous agitation. (2) Species bound to sulphide. The residue from (1) was leached at a temperature of 90 8C with 15 ml of HNO3 (1:7 V/V, pH 5.0). Continuous agitation was maintained for about 0.5 h, which to ensure complete extraction. (3) Species bound to organic matter. three millilitres of 0.02 mol/l HNO3 and 10 ml of 30% H2O2 (adjusted to pH 2 with HNO3) was added to the residue from (2), and the mixture was heated to 85 8C for 2 h with occasional agitation. (4) Stable species. The residue from (3) was dissolved by the mixture of HF–HClO4. 3. Results and discussion 3.1. The content and the occurrence mode of mercury, arsenic and selenium in coal It is necessary to determine the amount of the volatile elements in the raw coals and the modes of their occurrence, in order to understand their volatilization behaviour during the UCG process. According to geochemistry, the mode of occurrence of trace elements can be classified as exchangeable form, bound to sulphide, bound to organic matter, or stable form. The exchangeable form refers to that bound to particulate surfaces through an adsorption–desorption process. Elements bound to sulphide are inorganic forms existing in different kinds of sulphur compound, especially pyrites. The trace elements bound to organic matter are soluble trace metals released when the organic matter is degraded. The stable form comprises primary and secondary minerals, which may hold trace metals within the crystal structure and are difficult to extract. Tables 2–4 show the content of mercury, arsenic and selenium in the coal samples and the partitioning of their modes of occurrence obtained through sequential chemical extraction. In these tables, ‘sum’ represents the accumulated value of the four modes and ‘total’ refers to the elemental content detected directly through digestion of the coal samples. It can be seen that the lignite has the highest content of Table 2 Mercury content and the mode of occurrence The mode of occurence Exchangeable Sulphide Organic Stable form Sum Total
0.01 0.30 0.01 0.07 0.39 0.38
2 77 2 19
ND 0.11 ND 0.13 0.24 0.25
– 46 – 54
ND 0.16 0.01 0.12 0.29 0.29
– 56 3 41
ND, not detectable.
S. Liu et al. / Fuel 85 (2006) 1550–1558 Table 3 Arsenic content and the mode of occurrence
The mode of occurence
in all the modes, and not only is it a thiophile element but it is also often associated with organic material. The sulphide form dominates in lignite while the organic form is greatest in the anthracite. The increase of the proportion of the organic form is beneficial to its vaporization from the raw coals.
Exchangeable Sulphide Organic Stable form Sum Total
ND 3.7 0.5 0.6 4.8 4.9
– 77 10 13
ND 1.5 0.2 0.6 2.3 2.4
– 65 9 26
ND 2.6 0.4 0.9 3.9 4.0
– 67 10 23
3.2. Volatilization of mercury, arsenic and selenium during UCG process
ND, not detectable. Table 4 Selenium content and the mode of occurrence The mode of occurence Exchangeable Sulphide Organic Stable form Sum Total
ND 1.5 0.4 0.5 2.4 2.4
– 62 17 21
0.2 1.5 0.7 0.8 3.2 3.2
6 47 22 25
0.3 1.7 2.4 1.2 5.6 5.7
5 30 43 22
ND, not detectable.
mercury, 0.39 mg/g, while that in bituminous coal and anthracite are relatively lower. The result of the sequential chemical extraction of mercury indicates that the mercury in coal occurs mainly in boundsulphide and stable forms, with a small quantity of exchangeable and organic forms detected, which is associated with pyrite and subsidiary sulphide . Lignite has the highest proportion of mercury in sulphide, which takes up 77%, and the total extracted component of mercury reaches 81%. The mercury bound to sulphide in bituminous coal and anthracite is close to 50% and the extracted component is only about 50%. The content of arsenic in the lignite is 4.8 mg/g while it is only 2.3 mg/g in the bituminous coal. Arsenic mainly occurs in sulphides and a certain quantity of the organic form was also detected. Arsenic bound to sulphide makes up 77, 65, and 67% of the total amount in the lignite, bituminous coal, and anthracite samples, respectively; it is always associated with pyrite in the form of arsenopyrite (FeAsS) or asenical pyrite (FeAsS2) . The proportion of the organic form is around 10% and the exchangeable form cannot be detected. Arsenic extracted by the sequential chemical extraction process ranged from 74 to 87%. The content of selenium in the coal samples follows the sequence of anthraciteObituminous coalOlignite; the anthracite is enriched in selenium. It is clear that selenium occurs
3.2.1. Characteristics of UCG of different coals With regard to the test coals, as expected, the lignite and bituminous coal were easily ignited and showed a high rate of temperature rise, whereas the anthracite was difficult to ignite; the ignition temperature of coal increases with the extent of coalification. A comparison of the UCG simulation tests of the different coals (oxygen-steam gasification process) is shown in Table 5, in which gas productivity is calculated based on total dry gas yield and the weight of consumed coal. The specific consumption refers to the amount of oxygen and steam needed to produce one standard cubic metre of gas. It can be observed for all three coals from the average gas composition data that for the oxygen-steam gasification process, with different steam and oxygen specific consumption, UCG can produce a medium heating value gas with a combined amount of hydrogen and carbon monoxide around 65%, even though the original coal compositions and reactivities were quite different. This is because the oxygen concentration on the surface of the coal seam is increased by injecting oxygen and coal combustion is intensified to sustain the high temperature field. Under these conditions, the reduction of carbon dioxide and steam are promoted to produce more hydrogen and carbon monoxide. In addition, the shift reaction in the presence of steam is also enhanced in the long gasification tunnel and provides a further increase in hydrogen. Variations in the gas components are about G5%, caused by the variation of the reaction area during the UCG process; this is quite different from surface gasification. Gas production from lignite is lower as its carbon content is less than 50%, which in turn relates to lower specific consumptions of oxygen and steam. In contrast, the specific consumption of oxygen for anthracite is high because its carbon content is more than 80%. The gas production from the bituminous coal is intermediate between that of lignite and anthracite. For UCG process, the gasification velocity involves the velocity of fire front along the gasification tunnel and that along the incline of the coal seam, which depends on the rate of oxygen supply, temperature as well as the shape and size of the tunnel. The gasification velocity along the gasification tunnel
Table 5 Comparison of UCG simulation tests of different coals Coal kind
Lignite Bituminous coal Anthracite
Average gas composition (%) H2
43.4 39.8 40.4
27.6 25.1 24.2
4.1 7.0 3.8
23.7 26.2 30.2
1.2 1.9 1.4
Gas productivity (m3/kg) 1.49 2.11 2.28
Specific consumption (m3/m3) O2
0.22 0.25 0.30
0.40 0.67 0.65
S. Liu et al. / Fuel 85 (2006) 1550–1558 Table 7 The content ratios of mercury, arsenic and selenium for char/coal and ash/coal Coal
Char/coal Ash/coal Char/coal Ash/coal Char/coal Ash/coal
0.60 0.14 0.60 0.06 0.59 0.12
1.53 4.48 1.33 6.07 0.97 4.33
0.98 1.01 0.89 0.96 0.75 0.18
Bituminous coal Anthracite
Fig. 2. Changes of lateral gasification velocity during UCG process.
(i.e. the lateral velocity), is calculated by dividing the distance between the high temperature points (taking 900 8C as the standard) by the time interval, based on the available temperature database. Changes of the lateral velocity during underground gasification of the coals are shown in Fig. 2. It can be seen that lignite gave the highest lateral gasification velocity, which reflects its high reactivity. The lateral gasification velocity of anthracite was smooth and lower due to its lower reactivity. A higher gasification velocity is obtained for bituminous coal at the beginning of UCG process but it decreased during the final stage of forward oxygen-steam gasification, as most of the coal volatiles had been released into the gas phase during the earlier stages. Also, the gasification velocity was reduced by coking of the bituminous coal. A higher lateral gasification velocity was observed at the positions beside the injection holes for all the coals because the gasification reaction was enhanced when the oxygen supply was close to the surface of the coal seam by moving the supply. At positions away from the oxygen injection points, a lower lateral gasification velocity was observed due to a decrease in gasification reactions caused by cavity growth. 3.2.2. Changes of mercury, arsenic and selenium content in UCG products The samples of UCG products, char and ash, were obtained from the UCG simulation tests for all three coals, and their proximate analyses are shown in Table 6, in which CT represents the carbon residue. It can be seen that the volatile and moisture contents obviously decrease whereas the ash Table 6 Analyses of char and ash samples (wt%) Coal
Char Ash Char Ash Char Ash
2.36 0.87 1.06 0.73 1.10 0.24
36.05 98.01 14.33 95.69 12.99 96.96
Bituminous coal Anthracite CT, carbon residue.
content increases in char samples compared with that in coals. Analyses of the ash samples show that only small quantity of carbon residue can be detected and the ashes produced from UCG hardly contains any residual combustible components. The levels of mercury, arsenic and selenium in UCG products, presented as ratios of char to coal and ash to coal are listed in Table 7. It can be seen that for mercury, the ratios of char to coal are all below 1 and the ratios of ash to coal lower than 0.2, which reflects the high volatilization of mercury from coal in the seam. With regard to arsenic, all the ratios except for anthracite char/coal are higher than one, and the ratios of ash to coal are even higher than four, indicating the enrichment of arsenic in UCG char and ash. For selenium, the ratios are all lower than 1 due to volatilization, except for lignite ash/coal. 3.2.3. Volatility of mercury, arsenic and selenium In order to understand the volatilization of mercury, arsenic and selenium during the UCG process, the volatility has been calculated based on the following formula, C V% Z 1K i Y !100 C0 in which, Ci represents the content of mercury, arsenic and selenium in the samples of char and ash (in mg/g); C0 is the content of mercury, arsenic and selenium in the coals (in mg/g), and Y is the yield of char or ash, as shown in Table 8, which can be calculated from the ash balance based on the proximate analysis of the char and ash samples. The calculated volatilities of mercury, arsenic and selenium during the transformation of coal to char and the conversion of char to ash are shown in Fig. 3. It can be seen that the volatilization of mercury, arsenic and selenium during UCG follows the sequence of HgOSeOAs, the same as reported previously for the combustion and gasification of crushed coal . Mercury has the highest volatility, which for coal to ash reaches more than 98%. The gross volatility of selenium from raw coal to ash is above 90%, but less than that of mercury. The volatility of arsenic is lower than 60% and arsenic enrichment has been found in UCG ash. It has also been found
Table 8 The yield of char and ash (wt%)
Lignite Bituminous coal Anthracite
41 52 88
11 7 11
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depend largely on the properties of the specific element, the mode of its occurrence, the coal reactivity, etc. 3.2.4. Influence of elemental properties Both mercury and arsenic have close association with sulphur-bearing minerals, mainly pyrite, but the volatility of mercury during UCG process is greater than that of arsenic, since mercury is a highly volatile element with a lower boiling point. On the other hand, with regard to UCG process, the heating of the coal seam proceeds slowly, which is not favorable for the evaporation of arsenic . Selenium has a much higher volatility than arsenic, which corresponds with the lower boiling points of their oxides  and is shown during the transformation of coal to char. 3.2.5. Influence of the occurrence mode The trace elements occurring in organic form and sulphide are easily released at high temperatures. The lignite has the highest proportion of mercury in sulphide form compared to the bituminous coal and anthracite and thus mercury shows the highest volatility from lignite. A proportion of selenium occurs in organic form, which is easily decomposed during the pyrolysis stage of UCG leading to its high volatility.
Fig. 3. Comparison of volatility of mercury, arsenic and selenium during UCG of different coals (a, transformation of coal to char; b, transformation of char to ash).
that the volatility of arsenic rises during the conversion of char to ash at high temperatures for gasification, and this corresponds to its increased volatilization from the coal in the seam with temperature. In contrast, arsenic volatilization during the combustion of crushed coal does not always increase with temperature . It is clear that coal type has an effect on the volatilization behaviour of the elements. With regard to the UCG of lignite, the majority of the mercury, arsenic and selenium evaporate during the transformation to char, while for anthracite, the main evaporation occurs in the conversion of char to ash during the high temperature gasification stage. Therefore, it can be stated that the volatilization of mercury, arsenic and selenium
3.2.6. Influence of coal type With regard to the influence of coal type, it is clear that during the transformation of coal to char, the elements show the highest volatility for lignite, with bituminous coal next and anthracite the lowest. This is in accordance with the sequence of gasification reactivity, in which ligniteObituminous coalO anthracite. For underground gasification of coal in the seam, its reactivity affects the breaking of the coal and in turn influences the extent of reaction and the decomposition of sulphides and other minerals, which is closely related to the volatilization of the elements. Lignite has higher contents of moisture and volatiles, which are readily released on heating. This in turn helps the coal seam break down to form a loosened structure which has an increased reaction area. On the other hand, the coarse pore structure in lignite favours cracking of the coal seam at high temperatures, enabling air and oxygen to diffuse to fresh coal surfaces and improve the reaction rates and trace elements volatilization. In contrast, anthracite has a compact structure with a lower reaction area and lower reactivity and it was considered unsuitable for gasification. But UCG field tests with anthracite in Xiyang County revealed that it was feasible and a project has run there for almost 3 years. Thus, the UCG of anthracite can be illustrated as follows: the coal in the seam is difficult to crack at the stage of pyrolysis, but when the temperature rises to over 900 8C, the temperature difference between inside and outside leads to expansion and cracking of the coal in the seam, which enlarges the reaction area making gasification possible. As a result, the volatilities of mercury, arsenic and selenium for UCG char are especially low and rise sharply during the conversion of char to ash at high temperatures. Bituminous coal shows high reactivity at the beginning stage of gasification because of its high volatile content. After the pyrolysis stage, its strong cokability will affect
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its gasification reactivity. However, in a UCG process, since the fire front moves forward along the gasification tunnel and the gas flows through the large section of the gasification tunnel, the coal remains in contact with the gasifying agent, sustaining the UCG process. On the other hand, the higher stability of bituminous coal is not favorable for coal cracking and this causes the reaction surface area to decrease which in turn reduces the gasification velocity at the final stage. Therefore, higher volatility of the trace elements was observed during the transformation of coal to char. The above results only indicate the tendencies for volatilization of mercury, arsenic and selenium during UCG. The volatile elements do not always remain in the gas phase when they are cooled, adsorbed onto surfaces, and redistributed into different phases as the gas flows through the long gasification tunnel and production well. This influence on the distribution of mercury, arsenic and selenium in the final UCG products needs further investigation, depending largely on the length of the passage before gas reaches the surface. It has been found that gas-phase mercury is about 20% of the total mercury in UCG products, lower than from surface gasification and coal burning . 3.3. Thermodynamic prediction for species of mercury, arsenic and selenium in UCG gas 3.3.1. Program and the input for calculations Equilibrium calculations are often helpful for prediction of the trace element partitioning during gasification and identification of the possible species in fuel gas. The thermodynamic equilibrium model used in this work was MTDATA Version 4.71 for Windows, a soft package, which is available commercially from UK National Physical Laboratory (NPL). The Scientific Group Thermodata Europe (SGTE) database was chosen as the most appropriate data, which provides the Gibbs Free Energy of each substance to be calculated as a function of temperature, enabling the software to perform calculations to determine the equilibrium species for a defined system. The chemical system can be defined in terms of the elements or compounds involved. In this work, the major components, e.g. C, H, O, N, S, Cl in coal were considered with the trace elements in UCG systems, mercury, arsenic and selenium being modeled. Mineral elements such as Ca, Fe, K, Na were not considered in this study as it has been proven in the thermodynamic equilibrium study of trace elements that the levels of these mineral elements in coal have hardly any effect on the volatilization and the distribution of trace element species. The calculations were performed for oxygen-steam gasification conditions at temperatures ranging from 0 to 1600 8C. For UCG in shallow coal seams in China, the underground gasifier is always constructed in existing production mines with large tunnel sections (about 2 m2 in field tests), and thus the gasification pressure is low and fluctuating with cavity growth and the moving oxygen supply at different gasification stages. In this study, the operation pressure of 0.12 MPa was used as the input for calculations for UCG in a shallow coal seam, although the effect of increased pressure was evaluated. The
Table 9 Input for calculations Element
Hg As Se C H O N S Cl
1.26!10K6 3.18!10K5 4.09!10K5 54.63 89.00 80.69 0.19 0.13 3.62!10K4
bituminous coal was used for the calculations and a complete list of the input parameters is provided in Table 9. 3.3.2. Equilibrium compositions of mercury, arsenic and selenium Fig. 4 shows the equilibrium compositions of mercury, arsenic and selenium in oxygen-steam gasification at 0.12 MPa. It is clear that mercury species at equilibrium are all distributed in gas phase. Hg(g) is the only gas species above 90 8C. HHg(g) is the main species at temperatures lower than 90 8C; HgO(g) and HgCl2(g) are not found. Thus, mercury always presents in the gas phase and is mostly emitted with the fuel gas. Even when the gas temperature decreases to 200– 300 8C at the production well, it will remain in the gas phase. The mercury concentration in UCG gas has been measured in the UCG simulation test with lignite. The results have confirmed that mercury exists mainly as Hg0(g) in UCG gas and that its content changes with gasification time . In fact, the mercury content in UCG gas is lower than that predicted from theory as part of the mercury is adsorbed by ashes and dust as the gas passes through to the production well. From the equilibrium data for arsenic, it can be seen that arsenic mainly exists as the solid phases, As and As2S3, when the temperature is lower than 350 8C. As2(g) forms above 300 8C and is the main gaseous species at temperatures from 350 to 1100 8C. AsS(g) appears above 600 8C and becomes the dominant species between 1100 and 1450 8C. AsO(g) and As(g) appear at 900 8C and become the primary species above 1450 8C. For the reduced species, AsH3(g) occurs between 300 and 1350 8C and AsH(g) is present between 750 and 1600 8C; these are present at lower levels at a pressure of 0.12 MPa. From the theoretical results, arsenic is distributed between all the gas species at the high temperatures for gasification. When the gas reaches the top of the production well its temperature has decreased to around 200–300 8C, at which arsenic will be in the condensed phases, As and As2S3. This suggests that arsenic will be re-distributed through chemical reactions to form solid substances as the gas temperature decreases, provided there are no kinetic barriers. The occurrence of FeAsS and FeAs2 between 300 and 500 8C has been predicted in surface gasification in the presence of ferrous oxide . Therefore, based on the thermodynamic equilibrium study, no gaseous arsenic species should occur in UCG gas as it condenses as the gas flows through the underground channel, even through most of the arsenic will
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condensed phase of selenium is in the form of Se when the gas temperature decreases to below 70 8C. Thus, it can be inferred that all the above gas species coexist during the gasification stage. H2Se(g) should be the only gaseous species as the gas cools and can be expected to be retained in the UCG gas between 200 and 300 8C. 3.3.3. Effect of pressure UCG can operate over a range of pressures up to geological fracture conditions. For UCG in a virgin deep coal seam (e.g. European UCG test), the pressure of the gasifier will be high since the controlling pressure for UCG becomes the hydrostatic pressure, which increases with the depth of the coal seam. Thus, the effect of increased pressure on equilibrium compositions of the elements was evaluated at the pressures ranged from 0.12 to 3 MPa. The results show that the variation of the pressure remarkably changes the proportion of the gaseous species of arsenic and selenium while Hg(g) is the stable gas species at different pressures. At a pressure of 3 MPa (as shown in Fig. 5), with regard to arsenic, the percentage of AsH3(g) increases remarkably and becomes the dominant species below 1250 8C while AsH(g) presents as the main gas species above 1250 8C. AsH2(g) is also predicted at high temperatures. The increased presence of the reduced species indicates that reducing atmosphere is enhanced at high pressures. Moreover, the
Fig. 4. Equilibrium compositions of (a) mercury, (b) arsenic and (c) selenium under oxygen-steam gasification at pressure of 0.12 MPa.
evaporate from the coal seam during the UCG process. This potential beneficial situation with arsenic needs to be confirmed in practice. Selenium can be seen to be present in the gas phase over almost the whole temperature range. However, changing temperature may slightly modify the relative proportions of the gas species. H2Se(g) is the primary species when the temperature is below 1570 8C. HSe(g) occurs above 800 8C and sharply increases with temperature, and so will coexist with H2Se(g) during gasification stage. In addition, Se(g), and SeS(g) to a lesser extent, are observed only at gasification stage. The
Fig. 5. Equilibrium compositions of (a) arsenic and (b) selenium under oxygensteam gasification at pressure of 3 MPa.
S. Liu et al. / Fuel 85 (2006) 1550–1558
percentage of AsS(g) decreases sharply, which shows that sulphide species are not preferred at high pressures. For selenium, at 3 MPa, the proportion of H2Se(g) rises, HSe(g) decreases, and other minor species disappear. H2Se(g) becomes the dominant gas species over the whole range of temperatures. In addition, it is clear that high pressure makes the condensation point of the elements increase. The condensed phase of arsenic, As, occurs at 430 8C at pressure of 3 MPa, whereas at a pressure of 0.12 MPa it occurs at about 380 8C. For selenium, the condensed species, Se, occurs around 160 8C at 3 MPa, 90 8C higher than at 0.12 MPa. This indicates that the volatile elements become easier to condense with increasing gasification pressure. 4. Conclusions The volatilization of mercury, arsenic and selenium from the coal in the seam during the UCG process follows the sequence of HgOSeOAs. Mercury and selenium show higher volatility of more than 90% under the simulation test conditions. The volatility of arsenic is below 60% and arsenic enrichment has been found in UCG ash. Arsenic volatilization from the coal in the seam is also enhanced by the temperature, which is different from the result of arsenic evaporation during the combustion of crushed coal. The volatilization largely depends on the element properties and is favoured by the occurrence of organic and sulphide forms in the coal. Coal type and its UCG behaviour also have an effect on volatilization. The higher the coal reactivity, the easier is the evaporation of the elements from coal in the seam during the transformation of coal to char. Hg(g) is the dominant species of mercury present in UCG gas. Arsenic is present as condensed phases (As2S3 and As are the possible) when gas temperature decreases to around 200–300 8C at the exit of the production well. For selenium, H2Se(g) is the main gaseous species at the temperatures lower than 1200 8C and can be retained in UCG gas. The condensed species of selenium will present as Se. The effect of the pressure on equilibrium was shown in the remarkable change of proportion of the species. High pressure leads to the enhancement of the reduced species and makes the condensation temperature of the volatile elements increase.
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